import java.util.*;

public class NullDemo13 {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add(null);

        List list1 = new LinkedList();
        list1.add(null);

        List list2 = new Vector();
        list2.add(null);

        Set set = new HashSet();
        set.add(null);

//        Set set1 = new TreeSet();
//        set1.add(null);// 报错

        Map map = new HashMap();
        map.put(null,null);

//        Map map1 = new Hashtable();
//        map1.put(null,null);// 值不让放null

//        Map map2 = new TreeMap();
//        map2.put(null,null);
    }
}
